When you encounter slow queries in a production-like environment, the first step is to identify the root cause. Common causes of slow queries include missing or inefficient indexes, unnecessary full table scans, poorly written joins, suboptimal filtering, and returning more data than necessary. The SQL EXPLAIN command is a powerful tool to help you diagnose these issues by showing how the database executes your query. By inspecting the query plan, you can see whether indexes are being used, how tables are joined, what type of scans are performed, and which steps are the most expensive in terms of cost and row estimates.

To troubleshoot a slow query using the EXPLAIN output, start by reading the plan from the bottom up. Look for steps that involve sequential scans (Seq Scan) on large tables, which often indicate missing indexes. Notice if hash joins or nested loops are used where a more efficient join could be possible with proper indexing. Pay attention to the estimated cost and row counts for each operation; high values may point to bottlenecks. If the plan shows multiple scans or redundant joins, consider rewriting the query for efficiency. Evaluate the filtering and ordering steps, as sorting large result sets or filtering without indexes can be expensive. By following this step-by-step approach, you can pinpoint the exact part of the query that needs optimization.